Shifting between speed ratios in an automatic transmission involves an exchange of on-coming and off-going fluid operated friction elements and is generally characterized as comprising three successive phases: a fill phase, a torque phase and an inertia phase. In the fill phase, the on-coming element is prepared for torque transmission; in the torque phase, the torque exchange occurs without a corresponding speed change; and in the inertia phase, the speed change occurs.
In shift controls based on an open-loop control philosophy, the fluid pressure supplied to the on-coming element during the torque and inertia phases is progressively increased in accordance with a predetermined pressure schedule. The scheduled pressure is used to generate an electrical drive signal, typically in the form of a PWM duty cycle, which is applied to an electro-hydraulic actuator. The actuator is connected to a source of fluid pressure and operates to develop a fluid apply pressure therefrom in relation to the drive signal.
The scheduled pressure values are calibrated to achieve optimum shift quality in a nominal vehicle, but actuator variability and clutch performance variations which occur over time may adversely affect the shift quality actually achieved. For this reason, adaptive control techniques have been employed to correct the predetermined pressure schedule, based on a measure of the deviation of the actual shift quality from the desired or optimum shift quality. An example of such a control is set forth in U.S. Pat. No. 4,653,350 to Downs et al., issued Mar. 31, 1987, and assigned to General Motors Corporation.
In practice, the adaptive pressure corrections are developed under specified intermediate pressure range operating conditions known to provide accurate, repeatable results, and then applied to other operating conditions by extrapolation. This produces beneficial results under most operating conditions, but tends to overpressure or underpressure the torque establishing devices under certain operating conditions due to the nonlinearity of the actuator error. Specifically, relatively little error occurs at the limits of operation (that is, 0% and 100% duty cycle), but considerable deviation may occur at intermediate duty cycle values. This phenomenon is graphically illustrated by the pressure vs. duty cycle traces 10 and 12 of FIG. 2, where trace 10 represents the idealized output pressure of the actuator over the range of PWM duty cycles, and trace 12 represents the actual relationship. Thus, it can be assumed that little or no actuator variability will occur in a shift performed at or near the limits of the duty cycle range.
However, a certain amount of adaptive correction is probably required even under operating conditions in which little or no actuator error should occur, since it is always necessary to compensate for variability of the torque establishing devices. But no information is available about the torque establishing device variability per se, since the adaptive pressure corrections inherently compensate for the combined variability of the actuator and the torque establishing devices. In other words, the adaptive pressure correction used at the limits of actuator operation will be inappropriate, at least to the extent that the correction amount is based on actuator variability.